Carbonless copy paper sets consist of a top sheet known as the CB (coated back) sheet, a back sheet known as the CF (coated front) sheet, and optionally one or more intermediate sheets known as CFB (coated front and back) sheets. The coatings of the back of the CB sheet, of the front and back of the CFB sheets if any, and of the front of the CF sheet contain materials that when brought into association with each other develop a coloured image.
When the front of the CB sheet is typed on or otherwise pressure imaged, material is transferred between the back of one sheet and the front of the next through the set to give rise to the copies required. Usually, a solution of a dye precursor or "colour former" carried on the sheet as isolated droplets each confined within a pressure rupturable barrier is transferred from the back of one sheet to the front of the next after rupture of the barrier by the applied pressure, and interacts with a "colour developer" present there to give the image.
All of this is very well known and requires no detailed description.